Fc receptor-like (FCRL) 4 is an immunoregulatory receptor expressed on a subpopulation of human memory B cells of mucosa-associated lymphoid tissue. Fc receptor function of FCRL4 was demonstrated by binding of IgA to FCRL4 following heat aggregation of the Ig. In this study, we demonstrate that FCRL4 recognizes J chain-linked systemic IgA in the absence of heat aggregation. We further demonstrate that mucosal secretory IgA is not recognized by FCRL4 and that systemic IgA binding can be competitively inhibited by recombinant secretory component protein. Finally, we provide evidence that primary FCRL4-bearing human memory B cells are constitutively bound to IgA. Our study provides a mechanism for the negative regulatory activity of FCRL4 on AgR-mediated B cell activation.
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